Upgrade reference plugin to version 0.2.3

Upgrade to the latest reference plugin, providing slightly refined
titlepage and admon graphics.

build.gradle

@@ -4,7 +4,7 @@ buildscript {
 	}
 	dependencies {
 		classpath("org.springframework.build.gradle:propdeps-plugin:0.0.1")
-		classpath("org.springframework.build.gradle:docbook-reference-plugin:0.2.2")
+		classpath("org.springframework.build.gradle:docbook-reference-plugin:0.2.3")
 	}
 }